Cellular wireless communication systems typically rely on coherent detection and link adaptation. For this, the receiver must be able to estimate the channel and possibly report back channel quality information. Hence, Reference Signals (RSs) known by the receiver, are provided by the transmitter to enable channel estimation in the receiver. Contemporary systems, such as Long Term Evolution (LTE) communication system, utilize Common cell-specific RSs (CRS) in the downlink, which are used both for demodulation purposes and for various channel quality estimations, such as Channel Quality Indicator (CQI), Precoding Matrix Index (PMI) and Rank Indicator (RI).
One CRS defines a so called antenna port; and one, two or four antenna ports are supported in the LTE downlink. The antenna ports can be mapped to actual physical antennas in a proprietary manner, which is an implementation issue and not standardized. The CRS structure in one Resource Block (RB) is illustrated in FIG. 1 where a RB is defined as Nsymb consecutive OFDM symbols in the time domain and Nsc consecutive subcarriers in the frequency domain. In FIG. 1, each square represents a Resource element (RE), and for example, in LTE one RB may consist of 12×7=84 REs; and there is the same RS structure in each RB in a cell. For the LTE uplink, a different solution is adopted not relying on CRSs. In this solution the RS has been divided into Demodulation RSs (DMRS) and Sounding RSs (SRS), where the former are contained only in the scheduled RBs, while the latter can be transmitted over the whole system bandwidth. With this distinction of two different types of RSs, the RS overhead can be kept on a reasonable level in the system.
As wireless communication systems evolve, additional features are added, for example, more transmit antennas. One such example is LTE-Advanced, which will support eight transmit antennas. Therefore, additional antenna ports which are associated with additional RSs need be defined. Either eight new antenna ports is defined; or four, six or seven new antenna ports are to be defined in addition to the legacy antenna ports, in LTE, to support backwards compatibility.
CRS overhead can become significant for multiple antennas, and therefore in LTE, antenna port two and three were given a RS density which is lower than that for antenna port 0 and 1 in order to reduce the CRS overhead. For the case of four antenna ports in LTE, the RS overhead is about 14%. If the same CRS design methodology is used for eight antenna ports, the CRS overhead will reach to 28%, which is not acceptable for a wireless communication system of this kind.
As the RS overhead will become an issue again for eight transmit antenna configurations, the principle from LTE uplink can be utilized; namely to define a number of low-density CRSs used only for measurements and define DMRS for demodulation purposes. The CRSs are cell-specific and can potentially span the whole system bandwidth, whereas the DMRSs are user equipment (UE) specific and are only present in scheduled RBs. The CRS is sometimes also referred to as a Channel Spatial Information-RS or Channel State Information-RS (CSI-RS). Since the requirements on demodulation performance are more stringent than for channel quality measurements, the RS density of the new CRS could be made much lower than the density of the existing CRS (for example, in LTE). In total, it could thus be expected that the RS overhead could be manageable.
According to a prior art solution, it was proposed that the CRSs for an LTE-Advanced eight transmit antenna system should not be spread over the whole bandwidth as in LTE, but be contained in particular CRS RBs. These CRS RBs should be located in the time- and frequency domain such that sufficient measurement performance can be obtained and their positions are known to the LTE-Advanced UE. It is also proposed that these RBs could shift its frequency position as time progresses, which is denoted as CRS hopping. In that way, the CRS could cover a larger part of the bandwidth over time.
According to another prior art solution, sparse CRSs confined to certain RBs (i.e., CRS RBs) was also considered. To mitigate inter-cell interference, a set of higher-layer configured cell-specific offsets (subcarrier offset, RB offset, sub-frame offset) is suggested. However, this prior art solution has not considered inter-cell interference mitigation without any assistance of higher-layer signalling, for example, as is the case in LTE.